Wet process phosphoric acid is a relatively impure product, sometimes called filter acid, made by an acidulation process in which phosphate rock reacts with sulfuric acid to produce phosphoric acid and gypsum. In the acidulation process, raw rock which is primarily a calcium phosphate is treated with an acid solution composed of about 95 percent phosphoric acid and 5 percent sulfuric acid. The reaction product is "filter acid" or about a 30-32 percent P.sub.2 O.sub.5 wet process acid which still contains about 1.5-2.5 percent of H.sub.2 SO.sub.4 and 1.5-4.0 percent of fluorine. In the further treatment, this acid is concentrated by evaporation to produce about 54 percent P.sub.2 O.sub.5 solution which is commercial wet process acid. During the concentration, most of the fluorine is vaporized as SiF.sub.4 and HF and must be recovered from the vapor by scrubbing with water. The solution obtained contains large amounts of silicofluoride and silica in the aggregate and deep ponds of this waste cover large areas. It is therefore worthwhile to remove the fluorine in a useful form by the process of this invention before the concentration step which will reduce atmospheric fluorine pollutants and cost of scrubbing. There are other advantages such as greater ease of carrying out the concentration and in providing a phosphoric acid for animal feed production. Such an acid must have a P/F ratio by weight of at least 50 and preferably 100.
Numerous efforts have been made to remove the fluorine at various stages in the manufacture of fertilizer from raw rock. These have usually involved a volatile fluoride or an insoluble fluorosilicate which create problems in themselves. There has not been a successful commercial method of defluorinating at the 30-32 percent P.sub.2 O.sub.5 stage except by precipitation of an alkali metal silicofluoride of low commercial value.
An analysis of a typical wet process phosphoric acid which may be defluorinated by the process of this invention is as follows:
______________________________________ Per Cent ______________________________________ Ca 0.14 Al 0.313 Si 0.496 F 2.52 SO.sub.4.sup. 2.sup.- 2.27 P.sub.2 O.sub.5 30.0-32.0 ______________________________________
It is clearly evident that this acid is quite impure and it meets no clearly defined specification. The proportion of P.sub.2 O.sub.5 varies, and the proportion of impurities is also quite variable. A considerable amount of sulfate and fluorine is present, the latter usually as a fluorosilicate although in the rock it is probably present as calcium fluoride. The P.sub.2 O.sub.5 is not all present as free phosphoric acid since other dissolved phosphate salts are probably present. Actually, there may be 10 to 15 percent less free acid than would be indicated by the P.sub.2 O.sub.5 content.